The invention relates to a method and a control device for operating a road-coupled hybrid vehicle, having an electronic control unit, a primary motor which is associated with a first axle, and a secondary motor which is associated with a second axle, it being possible to switch between the primary axle and the secondary axle, depending on the operating strategy. The terms “primary motor” or “secondary motor” may also be understood to mean a primary or secondary drive unit, respectively, having multiple motors, such as wheel-specific drives.
Various methods for operating a road-coupled hybrid vehicle are described, for example, in German patent applications 10 2012 211 920 or 10 2013 208 965, not previously published.
German patent application 10 2012 211 920 is directed to a so-called “road-coupled” hybrid vehicle having at least one primary motor (an electric motor, for example) which acts as a drive motor on a first axle of the hybrid vehicle, and having a secondary motor (an internal combustion engine, for example) which acts as a drive motor on a second axle of the hybrid vehicle. The primary motor and the secondary motor are not coupled via a clutch, but, rather, are drive-coupled by means of the roadway via the wheels. Road-coupled hybrid vehicles of this type are also referred to as “split-axle” hybrid vehicles. An electric motor is preferably used as the primary motor, and an internal combustion engine is preferably used as the secondary motor.
Hybrid vehicles of this type are operable in particular in a first operating mode (preferably E mode for purely electric travel), in which the primary motor alone is operated for the drive control, and in a second operating mode (auto mode), in which the secondary motor may also be automatically switched on and off for the drive control.
German patent applications 10 2012 211 920 and 10 2013 208 965 relate to methods for switching on the secondary motor.
The object of the invention is to improve a hybrid vehicle of the above-mentioned type with regard to its efficiency as well as its driving stability.
This and other objects are achieved according to the invention by a method, and a corresponding control device, for operating a road-coupled hybrid vehicle, having an electronic control unit, a primary motor which is associated with a first axle, and a secondary motor which is associated with a second axle. By way of the electronic control unit, the primary motor and the secondary motor are basically controlled in a drive-oriented manner such that preferentially only a single-axle drive by the primary motor is provided. Beginning with the secondary motor switched off, when a traction requirement is determined, the secondary motor is switched on regardless of whether the vehicle is traveling on a curve. And, beginning with the secondary motor switched on, when a traction requirement is not present, the secondary motor is not switched off until travel by the vehicle on a curve is not recognized.
By means of the electronic control unit, the primary motor and the secondary motor are basically controlled in a drive-oriented manner in such a way that preferentially only a single-axle drive by the primary motor is provided.
In particular within the scope of hybrid vehicles, the term “drive-oriented control” is understood to mean not only a short-term efficiency-oriented control which solely minimizes fuel consumption, but in general also a longer-term control, oriented toward maximum availability of the requested overall power and/or electrical power, of the drive components and/or the axle-related drive torque distribution. Drive-related controls of the drive components in hybrid vehicles are also already known per se as so-called hybrid operation strategies.
When, beginning with the secondary motor switched off, a traction requirement is determined, according to the invention the secondary motor is switched on, regardless of whether the vehicle is traveling on a curve.
Alternatively or additionally, beginning with the secondary motor switched on, when a traction requirement is not, or is no longer, needed, the secondary motor is not switched off until it is determined that, in addition, the vehicle is not, or is no longer, traveling on a curve. In the present context, in the broadest sense the term “traveling on a curve” is understood to mean that even fairly small turning radii, such as during a lane change, may be included.
A traction requirement is preferably determined when an expected traction requirement in the longitudinal direction is predicted when wheel slip is recognized or when the ASC control is active.
An expected traction requirement in the longitudinal direction is predicted when a requested longitudinal dynamic setpoint parameter (in particular, driver input longitudinal acceleration) exceeds a defined threshold value based on a longitudinal dynamic potential parameter (in particular, traction-related maximum possible longitudinal acceleration).
The invention is based on the following further considerations.
According to the invention, starting from a basically prioritized default of the first operating mode (preferably E mode for purely electric travel) on the drive control side, in which the primary motor alone is to be operated, (by way of exception) the secondary motor is additionally started in a preparation phase on the driving stability control side. The preparation phase is initiated when, based on a model-based prediction, an expected traction requirement has been anticipatorily determined. In the model-based prediction, an expected traction requirement is determined at least when, based on driver input (in particular, when the accelerator pedal is depressed), a requested longitudinal acceleration (or a variable proportional thereto) in the longitudinal direction is computed which is greater than a threshold value that is based on a computed longitudinal potential variable. This starting of the secondary motor by way of exception, beginning from the first operating mode with the secondary motor switched off, is also referred to below as a “switch-on request.” The switch-on request may also be made when slip between the axles is recognized, and/or when an ASC control is active. The switch-on request is fulfilled regardless of whether the vehicle is traveling on a curve.
The proportional longitudinal potential variable is equal or proportional to the maximum possible longitudinal acceleration, and is computed in such a way that in particular the instantaneous coefficient of friction and the inclination of the roadway as well as the vehicle mass are taken into account. This longitudinal potential variable may be determined, for example, using the Kamm's circle according to German patent application 10 2013 208 965 (having U.S. counterpart Ser. No. 14/277, 917), the subject matter of which is incorporated by reference herein.
If, beginning with a started secondary motor, which may be started based on either a prior switch-on request or the presence of the second operating mode (auto mode, for example) on the drive control side, a traction requirement is not, or is no longer, recognized, the secondary motor is not switched off until the vehicle is not, or is no longer, traveling on a curve. This delay according to the invention in switching off the started secondary motor until the end of a curve is also referred to below as “jolt prevention.” Jolt prevention refers to the prevention of an acceleration jerk, which is undesirable for relieving strain on the driver while traveling on a curve.
In addition, a distance- and/or time-related condition may prevent a jolt in order to avoid undesirable toggling between the drive modes.
In summary, as a result of the invention, the drive control having a preferred single-axle drive (in particular, purely electric travel) is basically prioritized by the primary motor, this priority being “overridden” by way of exception by a dual-axle drive when, firstly, beginning with the secondary motor switched off, a longitudinal acceleration-based traction requirement is expected (first exception case: “switch-on request”) and/or when, secondly, beginning with the secondary motor switched on without a traction requirement, travel on a curve has not ended (second exception case: “jolt prevention”).
In the first exception case (switch-on request), the traction in the longitudinal direction is preferred, regardless of whether the vehicle is traveling on a curve, in particular to provide sufficient drive force. In the second exception case (jolt prevention), the reproducibility or constancy of the driving characteristic is preferred in order to simplify lateral control for the driver while traveling on a curve.
The invention is preferably applicable when electric travel is to be preferentially carried out on the drive control side for minimizing CO2 emissions, i.e., when the primary motor is an electric motor. Due to the anticipatory determination of the expected traction requirement and the preparation phase (prior to the actual traction control), the invention is also particularly advantageous when the secondary motor is a motor which requires a certain time to start, such as an internal combustion engine.
For drive-oriented travel, the primary focus is on drive strategies which optimize driving performance and fuel consumption for controlling the primary and secondary motors. In contrast, for travel which is oriented toward driving dynamics, in particular when there is a traction requirement, the primary focus is the drive torque distribution on the axles.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.